The present invention relates to a scanning optical system which is used as an optical system for a scanning optical device such as a laser beam printer. Particularly, the present invention relates to the achromatic scanning optical system.
The scanning optical device deflects beams emitted from a light source such as a laser diode by means of, for example, a polygonal mirror, and converges the beam to form a spot on a surface to be scanned such as a surface of a photoconductive drum, through an f.theta. lens (scanning lens). The beam spot formed on the surface to be scanned moves (i.e., scans) on the surface in a predetermined scanning direction as the polygonal mirror rotates. In this specification, the direction in which the beam spot scans is referred to as a main scanning direction. Further, a plane including the scanning beam scanning in the main scanning direction is referred to as a main scanning plane. Furthermore, a direction perpendicular to the main scanning lane is referred to as an auxiliary scanning direction.
The f.theta. lens is provided with a single lens element or plurality of lens elements, and various types of aberrations are corrected such that the spot on the surface to be scanned scans linearly in the main scanning direction, at a constant speed on the surface to be scanned as the polygon mirror rotates at a constant rotation speed.
Recently, in order to increase an image forming speed, there has been developed a multi-beam scanning device which is provided with a plurality of light sources, such as laser diodes. The plurality of laser diodes emit a plurality of beams to form a plurality of scanning lines simultaneously. In a scanning optical system employed in such a multi-beam scanning device, positional relationship between the scanning lines formed by the plurality of scanning beams should be adjusted accurately such that a plurality of scanning lines are apart from each other by a predetermined distance.
In such a multi-beam scanning device, generally, wavelengths of the beams emitted by the plurality of laser diodes distribute, for example, within a range of a standard designed value .+-.15 nm. Therefore the wavelengths of two laser diodes forming the adjacent scanning lines may be different by 30 nm at the maximum. It the f.theta. lens has a lateral chromatic aberration, a writing start position along the main scanning direction, from which the scanning beam spot contributes to image formation, and a writing complete position, which is the end of the image portion on the scanning line, may differ between a plurality of lines, which may exceed an allowable range, and affects the quality of formed image.
Conventionally, the chromatic aberration of the f.theta. lens is compensated by combining a positive lens and a negative lens having different dispersion. Alternatively, the chromatic aberration due to variation of wavelengths emitted by the respective laser diodes may be reduced by selecting laser diodes which emit laser beams having closer wavelengths, i.e., by selecting laser diodes such that the emission wavelengths thereof distribute within a smaller range.
In order to correct the chromatic aberration of the f.theta. lens by selecting a lens materials (glass materials) having different dispersion as in the prior art described above, the number of lens elements of the f.theta. lens increases when compared with a case where the chromatic aberration is not corrected. In addition, in order to compensate for the chromatic aberration, lens materials cannot be selected only by their refractive indexes, and types of available lens materials are limited, thereby degree of freedom in designing the lens is lowered.
On the other hand, the selection of the laser diodes takes much time, and difference in the emission wavelengths due to variation in used periods o0 the light sources cannot be overcome.